Since the proposals to form an electrocatalytic coating material of platinum-group metal oxides (see U.S. Pat. No. 3,711,385) and mixed crystals or solid solutions of co-deposited oxides of platinum-group metals and film-forming metals (see U.S. Pat. No. 3,632,498), dimensionally stable electrodes of this type have revolutionized the chloralkali industry and have become widely used in other applications. Nevertheless, the search for a commercially-viable dimensionally stable electrode for use in oxygen-evolution conditions and which uses only minor amounts of noble metals, is still going on.
Although it has been known from U.S. Pat. No. 3,711,385 that the electrocatalytic coating of a platinum-group metal oxide could be made quite thin (said patent claiming a minimum thickness of 0.054 micron), in practice it has been found that to achieve any acceptable lifetime, or in some instances for the electrode to work at all, a somewhat thicker coating was necessary. Hence, usually ten to twenty thin coatings of a suitable paint solution are applied to the film-forming metal base and heated each time to give an electrocatalytic coating formed from the decomposed components of the paint containing about 5 to 20 grams by metal of the platinum-group metal oxide per square meter of the electrode area (i.e. its projected, geometrical surface area).
Many attempts have been made to economize on the precious metal content of these coatings, usually by partly replacing the platinum-group metal oxide by a compatible non-precious metal oxide such as tin dioxide (see for example U.S. Pat. No. 3,776,834) or tin and antimony oxides (see for example U.S. Pat. No. 3,875,043).
At present, probably the best electrode for oxygen-evolution is that described in UK Patent Specification No. 1 399 576, having a coating containing a mixed crystal of tantalum oxide and iridium oxide. However, known electrodes of this type contain at least about 7.5 g/m.sup.2 of iridium so that despite their excellent performance in terms of over-voltage and lifetime, the high cost of iridium makes these electrodes less attractive and in order to be competitive with cheaper anodes they must be operated at a relatively high anodic current density which necessitates various expedients in the cell design. Consequently, anodes made of solid lead, lead alloys, cobalt-silicon alloys and so forth are still used in many electrowinning plants despite the known disadvantage of such materials.
Another type of electrode proposed in UK Patent Specification No. 1 463 553 has a base which consists entirely of or at least at its surface of an alloy of a film-forming metal and an activating metal for instance a platinum-group metal, whose surface is oxidized during use or is preactivated by an oxidizing treatment to form in the outer part of the alloy a surface oxide layer to a depth of 1 to 30 microns. Such alloys have shown promise for electrowinning but are quite difficult to prepare by sintering or in another manner and are quite expensive because of the quantity of platinum-group metal in the alloy. Also, the pre-activation methods are difficult to control to obtain an improvement in the electrode performance.
There have also been suggestions for coating oxygen-evolving anodes with non-precious metal oxides such as manganese dioxide, usually in quite large quantities, and possibly with some additives: see for example, U.S. Pat. No. 4,072,586. The MnO.sub.2 coating is sometimes deposited over an intermediate conductive layer of, for example, tin and antimony oxides (U.S. Pat. No. 4,028,215) or on a titanium surface pretreated with a small quantity of RuO.sub.x (see Japanese published patent application No. 11753/80, Application No. 156740/76 and Electrochimica Acta, 1978, Vol. 23, pp. 331-335). Again, some of these MnO.sub.2 -coated electrodes have shown promise for electrowinning processes but have not yet met with commercial success.
The scientific literature has manufacturing of the passive surface films formed on film-forming metals, as well as such films doped with a small quantity of platinum metal or oxide, by cathodic deposition of platinum metal onto a cleaned titanium base, followed by anodisation. (See papers "Electronic properties of doped passive layers on titanium electrodes" by U. Stimmung and J. W. Schultze and "Investigations of doped passive layers on titanium electrodes by electron spectroscopy" by D. Hofman and U. Stimmung, presented at the ISE Budapest Meeting, Aug. 28 to Sept. 2, 1978). However, the results indicate that such PtO.sub.2 -doped films are almost insulating and the thus-produced platinum dioxide-doped films have a conductivity approaching that of metallic platinum only when excess platinum is present.
It has also been suggested in W. German Offenlegungsschrift No. 26 52 152 to form an electrode by anodically growing a film of titanium oxide on a titanium strip in an electrolyte containing a solution of platinum metal so as to occlude particles of platinum in the titanium oxide film. However, this procedure has not led to the production of useful electrodes.
One situation in which known electrodes have been particularly subject to failure and/or poor performance is the electrolysis of manganese-contaminated electrolytes where deposits of manganese or manganese oxide on the anode have led to "poisoning" of the electrocatalyst and rise of the cell potential. Other critical situations are where the cell is subject to shutdown, or current reversal which may lead to dramatic failure of coatings which, in other respects, has performed quite well.
An object of the invention is therefore to provide a film-forming metal electrode which is made electrocatalytic on its surface in an inexpensive manner, has a low oxygen overvoltage, is able to withstand cell shutdown, and even current reversal and, in addition, has an excellent resistance to the effects of manganese/manganese dioxide deposition.